The invention relates to a process for the recovery of sulfur from an acid gas stream that comprises hydrogen sulfide. One aspect of the invention relates to a sulfur recovery process that includes both the catalytic and biological conversion of hydrogen sulfide to yield a sulfur product and a gas stream having a low concentration of hydrogen sulfide.
The removal of sulfur from process gas streams can be desirable or even necessary for a variety of reasons including, for example, the need to remove sulfur compounds from the process steams of chemical and hydrocarbon processing plants in order to comply with government regulations.
One well-known method that is used to treat certain process streams that contain hydrogen sulfide to recover elemental sulfur is the Claus process. The Claus process is a two-step process that includes a thermal step followed by a catalytic step. In the thermal step, the hydrogen sulfide of a feed stream is partially oxidized by combustion with oxygen to form a combustion gas containing sulfur dioxide. The chemical reaction of the thermal step is represented by the following equation (1):2H2S+3O2→2SO2+2H2O  (1)The hydrogen sulfide and the formed sulfur dioxide contained in the combustion gas can undergo the Claus reaction whereby they are reacted to form elemental sulfur in accordance with the following equation (2):2H2S+SO23S+2H2O  (2)Further in the Claus process, unreacted hydrogen sulfide and sulfur dioxide in the combustion gas are catalytically reacted in accordance with the Claus reaction equation (2) by passing the combustion gas over a Claus catalyst, which provides for a lower Claus reaction temperature. The Clause process also provides for the recovery of the formed elemental sulfur as a product and for the yielding of a Claus tail gas.
While the Claus process is very effective at providing for the recovery of a major portion of the sulfur in its feed stream, it still only provides for up to about 94 to 96 percent sulfur recovery with a two-bed catalytic Claus plant. Claus plants with three or more catalytic beds can provide for incremental improvements in sulfur recoveries, but the practical upper limit of sulfur recovery with a Claus plant alone is upwardly to about 97 to 98 percent. The tail gas from a Claus process, however, can further be treated so as to provide for the conversion of the residual hydrogen sulfide and sulfur dioxide and the recovery of additional sulfur. With Claus tail gas treatment, e.g., the SCOT process, the overall sulfur recovery can approach upwardly to about 99 to 99.8 percent.
There is an ongoing need for improved sulfur recovery processes that provide for high sulfur recovery and better operating efficiencies preferably with lower capital costs. With increasingly more stringent sulfur emission standards, there is also a need for sulfur recovery processes that provide for even greater sulfur recoveries from process streams containing sulfur compounds than are provided by conventional sulfur recovery systems that include a Claus unit coupled with a Claus tail gas treatment unit.
It is thus an object of the inventive process to provide for a high sulfur recovery from a process stream containing a sulfur compound.
Another object of the invention is to provide a process for efficiently recovering sulfur from a process steam containing a sulfur compound.
Accordingly, one embodiment of the invention includes a sulfur recovery process. In this process an acid gas stream comprising hydrogen sulfide is charged as a feed to a sulfur recovery system operated so as to yield a first sulfur product and a Claus tail gas comprising hydrogen sulfide and less than about 1000 ppmv sulfur dioxide. The Claus tail gas is then charged to a biological gas desulfurization system operated to yield a second sulfur product and a sweet gas comprising less than 100 ppmv hydrogen sulfide.
Another embodiment of the invention includes a process for the recovery of sulfur from an acid gas stream. This process includes a Claus sulfur recovery step in combination with a biological sulfur recovery step to provide a sweet gas stream having a very low concentration of hydrogen sulfide and sulfur dioxide. The acid gas steam is reacted with oxygen under such oxidation conditions to yield a combustion gas comprising hydrogen sulfide and sulfur dioxide so as to have a ratio of hydrogen sulfide to sulfur dioxide exceeding 2:1. The combustion gas is reacted under Claus reaction conditions to yield a reaction gas comprising sulfur. Sulfur is recovered from the reaction gas to yield a tail gas comprising a concentration of hydrogen sulfide and less than 1000 ppmv sulfur dioxide. The tail gas is contacted with a lean absorbent thereby remove from the tail gas a portion of the hydrogen sulfide contained therein and to yield a sweet gas and a rich solvent comprising dissolved hydrogen sulfide. The dissolved hydrogen sulfide of the rich solvent is biologically oxidized to elemental sulfur by contacting the rich solvent with sulfur bacteria under suitable biological oxidation conditions with the rich solvent.
A yet another embodiment of the inventive process includes passing an acid gas stream comprising hydrogen sulfide to a combustion zone for partially oxidizing the hydrogen sulfide in the acid gas stream with oxygen to form sulfur dioxide thereby providing a combustion gas stream comprising sulfur dioxide and hydrogen sulfide. The amount of hydrogen sulfide oxidized in the combustion zone is controlled such that less than 33 volume percent of the hydrogen sulfide in the acid gas stream is oxidized to sulfur dioxide. The combustion gas stream is then passed to a Claus reaction zone operated under Claus conversion conditions to yield a reaction gas comprising sulfur. Sulfur is recovered from the reaction gas to yield a Claus tail gas comprising a concentration of hydrogen sulfide. The Claus tail gas is passed to an absorption zone for contacting the Claus tail gas with a lean caustic solution whereby hydrogen sulfide is recovered from the Claus tail gas and from which is yielded a sweet tail gas and a rich caustic solution. The rich caustic solution is passed to a bioreaction zone for the biological oxidation of the dissolved sulfide in said rich caustic solution to elemental sulfur.
Other objects and advantages of the invention will become apparent from the following detailed description and appended claims.